In light of the problem of ozone layer depletion that has become a focus of attention in recent years, the restrictions on CFCs (chlorofluorocarbons) and HCFCs (hydrochlorofluorocarbons) that are used as refrigerants in conventional refrigerating machines have become more stringent, and HFCs (hydrofluorocarbons) are coming into use as substitute refrigerants.
Mineral oils or hydrocarbon oils such as alkylbenzenes have been preferred for use as refrigerating machine oils when CFCs or HCFCs are used as refrigerants, but since changing the refrigerant can cause the refrigerating machine oil used with it to exhibit unpredictable behavior in terms of its compatibility with the refrigerant, its lubricity, its dissolved viscosity with the refrigerant and its thermal and chemical stability, it has been necessary to develop different refrigerating machine oils for different refrigerants. Examples of refrigerating machine oils that have been developed for HFC refrigerants include polyalkylene glycols (see Patent document 1), esters (see Patent document 2), carbonic acid esters (see Patent document 3) and polyvinyl ethers (see Patent document 4). Of the refrigerating machine oils mentioned above, esters are most widely used for refrigerators and air conditioning units.
Among HFC refrigerants, HFC-134a, R407C and R410A are routinely used as refrigerants for automobile air conditioners, refrigerators and room air conditioners. However, while these HFC refrigerants have ozone depletion potentials (ODP) of zero, their high global warming potentials (GWP) have led to their gradual restriction. It has therefore become an urgent issue to develop refrigerants as substitutes for such HFCs.
In light of this background it has been proposed to use, as substitute refrigerants for HFCs, fluoropropene refrigerants which have very low ODP and GWP, are noncombustible and are comparable or superior to HFCs in terms of their thermodynamic properties, as a measure of refrigerant performance. There has also been proposed the use of refrigerant mixtures of fluoropropene with saturated hydrofluorocarbons, C3-5 saturated hydrocarbons, dimethyl ether, carbon dioxide, bis(trifluoromethyl)sulfide or trifluoroiodomethane (see Patent document 5). In addition, difluoromethane refrigerants (HFC-32) are attracting attention as HFC refrigerants with relatively low global warming potential and high refrigerating efficiency.
Incidentally, one of the performance aspects required for a refrigerating machine oil is compatibility with refrigerants used in combination. Advances are therefore proceeding in the development of refrigerating machine oils suited for difluoromethane refrigerants or unsaturated fluorinated hydrocarbon refrigerants, when such refrigerants are to be used.
For example, a refrigerating machine oil serving to lubricate a refrigerant compressor generally must have compatibility with refrigerants in order for the refrigerating machine oil to be circulated in the cycle together with the refrigerant. However, when refrigerating machine oils that are conventionally employed as HFC refrigerants are used with difluoromethane refrigerants, the compatibility between the refrigerant and refrigerating machine oil is not sufficient, and the refrigerating machine oil ejected from the refrigerant compressor tends to pool in the cycle, resulting in a reduced amount of refrigerating machine oil in the refrigerant compressor and thus poor lubrication, and blocking of the expansion mechanism including capillaries. Refrigerating machine oils for difluoromethane refrigerants are being developed with the goal of avoiding this phenomenon, and for example, ester-based refrigerating machine oils have been proposed, including the refrigerating machine oils for difluoromethane refrigerants disclosed in Patent documents 6 to 12.